Firstly, I'm sorry about
your son's diagnosis. It's sh!t when a child gets this crappy disease. But also, I just wanted to second Charlie's suggestion of enteral nutrition. With a child, I would try that before I would try steroids. He doesn't need to be on said diet permanently because, let's face it, it's not a whole lot of fun for a child. But it's worth trying for a few weeks/months just to (hopefully) get the immediate flare-up under control.
Again, I understand your doubts about
the Humira but it sounds like your boy has severe disease to be honest :-/ For whatever it's worth, Humira is safer than long-term prednisone: a lot, lot safer.
Meanwhile, if you need to convince your GI that enteral nutrition is the way to go, here is an article you can print off to show him.
/www.ncbi.nlm.nih.gov/pmc/articles/PMC4462720/